1. Field of the Invention
The present invention relates to magnetic metal powder and its manufacturing method.
2. Description of Related Art
The manufacturing method of metal powder can be classified by its starting raw material. In other words, metal powder can be manufactured from its gaseous phase, liquid phase and solid phase. And, as a specific method for manufacturing metal powder from the gaseous phase, the known methods are a chemical vapor deposition (CVD) method, sputtering method and vacuum deposition method. As for methods of manufacturing metal powder from the liquid phase, the known methods are a co-precipitation method, gas or water atomization method, spray method and spray pyrolysis method. As for making metal powder from solid phase, there is a pulverizing method that uses a crusher to pulverize metal nuggets into particles of appropriate sizes or administering a prescribed process on the pulverized powder.
Various parts used in the electronics field will be more frequently and widely used in the high frequency range. The same can be said about printed circuit boards. Substrates with various characteristics will be in demand such as those with high or low dielectric constant, high magnetic characteristics or those that absorb radio waves. To obtain these substrates, magnetic powder with excellent high frequency characteristics are being mixed and dispersed into printed circuit boards according to its needs. Some of the magnetic powders being used are ferrite powder and carbonyl iron powder for high frequency use. In areas other than printed circuit boards, there is the packaging category where radio wave absorbing powders are mixed and dispersed within resin. In the field of conductive pastes, conductive particles are mixed and dispersed in thick film pastes to manufacture electronic circuits, resistors, capacitors and IC packages. Moreover, in soft magnetic materials, magnetic powder is used widely for making coil materials for power supplies like choking coils. As for magnetic materials, there are core materials for motors. Magnetic powder is also used in magnetic resistors and magnetic sensors.
A technique for creating metal powder for thick film paste using the spray pyrolysis method is known. This technique entails spraying a solution containing metal salts to create liquid droplets, and heating the droplets at a temperature higher than the metal salt decomposition temperature and at a temperature higher than the metal melting point, but if the metal forms an oxide at temperature below its melting point, at a temperature higher than the oxide decomposition temperature, in order to thermally dissolve the metal salt and melt the metal particles thus created.
According to the spray pyrolysis method, the metal powder thus obtained is spherical with excellent crystallization properties and with high dispersant characteristics. According to the spray pyrolysis method, for example, Ag powder can be formed with the maximum particle size of 1.7 xcexcm and the minimum particle size of 0.5 xcexcm using a solution containing AgNO3; Agxe2x80x94Pd alloy powder is formed with particle sizes ranging from 2.5 xcexcm (max) to 1.5 xcexcm (min) by using a solution containing AgNO3 and Pd (NO3)2, and Au powder is formed with particle sizes ranging from 1.0 xcexcm (max) to 0.5 xcexcm (min) using a solution containing HAuCl4. Also, these powders are said to have excellent crystalline characteristics.
In this manner, metal powder with particle sizes ranging from 0.5 to 2.5 xcexcm and excellent crystalline characteristics can be obtained. Metal powder with these properties is suitable as conductive paste.
However, the examples described above pertain to Ag, Agxe2x80x94Pd alloy and Au, but not to metal powder, especially Fe powder, that is suitable for using the mixing and dispersing of magnetic powder.
Prior art teaches methods of manufacturing metal powder by the spray pyrolysis method, and suggests the possibility of manufacturing Fe powder or Fe alloy powder. However, we have not as yet seen an example of actually manufacturing Fe powder or Fe alloy powder. In other words, it can be said that metal powder that can be manufactured by the spray pyrolysis method had imposed considerable restrictions on the types of metal powder.
It is noted that Fe powder or Fe alloy powder can be manufactured from gaseous phase and solid phase as explained above. However, the particle size of metal particles formed by the gaseous phase manufacturing method is very small, and thus, unsuitable to be mixed with resin. Also, metal powder formed from the solid phase manufacturing method has poor particle distribution and the shape of the powder particles is not spherical because crushing machines are used.
Thus, magnetic metal powder, especially Fe or Fe alloy powder with properties suitable to be mixed with resin were unavailable from conventional metal powder manufacturing methods.
The present invention relates to a manufacturing method to obtain magnetic metal powder with properties suitable to be mixed with resin, and to provide novel magnetic metal powder that was previously unavailable.
In order to solve the problems described above, the inventors of the present invention studied the causes that restricted the types of metal powder that could be produced under the spray pyrolysis method. The spray pyrolysis method uses liquid solutions as raw material, and consumes thermal energy for pyrolyzing water unrelated to the target metal sought during the high temperature processing step. Also, because water vapor is generated, the environment for performing the thermal pyrolysis, or typically, the reducing process, becomes a vaporous atmosphere. The moisture in the water vapor atmosphere diminishes the reducing operation. Therefore, depending on some of the conventional spray pyrolysis methods, it is believed that metal powder that uses starting material requiring strong reduction could not be obtained. The Ag, Agxe2x80x94Pd alloy and Au noted above can be obtained without requiring a strong reducing power.
The inventors were successful in manufacturing spherical-shaped single crystal Fe powder, which was unobtainable under conventional methods, by conducting a heat treatment on dry compound powder with specified particle sizes, as the starting raw material, without using the wet starting material as in the case of the spray pyrolysis method.
In accordance with one embodiment of the present invention, a method for manufacturing magnetic metal powder includes a raw material supply step to supply raw powder for forming magnetic metal through pyrolysis with a carrier gas to a predetermined heat processing region, a heat treatment step for heating the raw powder at a temperature higher than the thermal decomposition temperature of the raw powder, and a cooling step in which a product obtained from pyrolysis is cooled to provide magnetic metal powder including the magnetic metal element.
In addition to the merit that spherical-shaped single crystal Fe powder, unobtainable under conventional methods, can be obtained under the present invention, the method requires less heating energy than that of conventional spray pyrolysis methods because the heat treatment is implemented on dry compound powder, and there is the additional benefit of a high recovery rate.
The magnetic metal powder obtained in accordance with the present invention is not limited to a single crystal form of Fe, but also allows the manufacturing of other magnetic metal powder. As for the magnetic properties, the present invention can be used to make soft magnetic materials as well as hard materials.
In accordance with the present invention, the carrier gas includes a reducing gas, and a magnetic metal powder can be obtained by reducing the raw powder in the heat treatment step with the reducing gas, and cooling down the reduced substance.
In accordance with the present invention, it is also possible to obtain a magnetic metal powder by first creating a melt from the reduced substance in the heat processing step and by re-crystallizing the melt at the cooling process step.
Moreover, the present invention allows reducing the melt created after melting the raw powder at the heat processing step, and obtaining a magnetic metal powder by re-crystallizing the reduced melt in the cooling process step. In other words, the present invention offers the option of using a method to form a melt of the raw powder and cool and solidify the melt, after reducing the raw powder in solid form, or a method to melt the raw powder in solid form into a molten state and reduce the melt while retaining the same in its molten state, and then cool the melt. In this manner, by melting the raw powder once, the magnetic metal powder to be obtained can be readily changed into single crystal form.
In present invention, a magnetic powder of pure iron may be obtained by using an iron oxide powder as the raw powder.
Also, in the process of manufacturing the magnetic powder, the present invention allows the formation of a coating layer on the surface of the magnetic powder. To form the coating layer, the raw powder and a powder formed from a compound consisting of at least one element as its ingredient with a reducing power stronger than that of the magnetic metal included in the raw powder may be supplied to the heat treatment region. In this case, the powder formed from a compound consisting of at least one element as its ingredient with a reducing power stronger than that of the magnetic material may preferably have particle sizes smaller than those of the raw powder. Also, the raw powder may contain a compound consisting of at least one element as its ingredient with a stronger reducing power than that of the magnetic metal, with the result that a coating can be formed on the surface of the magnetic powder during the process of manufacturing the magnetic powder. Methods of forming the coating layer shall be explained later.
As explained above, the present invention provides Fe powder or Fe alloy powder with properties unavailable under conventional methods. That is, the present invention concerns a method comprising the steps of supplying a powdered oxide of at least one type selected from Fe group elements with a mean particle size of about 0.1-100 xcexcm in a heat treatment atmosphere, forming a melt of the powdered oxide in the heat treatment atmosphere, and cooling and solidifying the melt to form magnetic metal powder composed of at least one type of Fe group elements. In the manufacturing method, a reducing step may be conducted in the heat treatment atmosphere before the melt is formed, or after the melt is formed but before it is cooled and solidified.
The magnetic metal powder of the present invention may have a mean particle size in the range of about 0.1-20 xcexcm. The mean particle size may preferably be from about 0.5 to 10 xcexcm, or more preferably from about 1 to 5 xcexcm. Moreover, excellent magnetic characteristics and high frequency characteristics can be obtained because the magnetic metal powder to be obtained by the present invention can be formed into a single crystal form.
In the method of manufacturing magnetic metal powder described above, it is possible to form a coated layer during its manufacturing process.
The powder obtained by the process of the present invention is a single crystal powder composed of Fe as a main ingredient. The powder obtained by the process of the present invention is novel magnetic metal material in a spherical form with a mean particle size ranging from about 0.1 to 20 xcexcm, which was unobtainable under conventional methods. A preferred mean particle size in the magnetic metal powder obtained by the present invention may range from about 0.5 to 10 xcexcm, and more preferably about 1 to 5 xcexcm. Also, the magnetic metal powder obtained from the present invention offers an excellent magnetic characteristic of more than 2.0 T in saturated magnetic flux density.
While the magnetic metal powder of the present invention can be formed only from the metal, it is also possible to form a coating layer on the surface of the magnetic metal powder. While the coating layer can be formed after the magnetic metal powder is made, it can also be formed during the manufacturing process of the magnetic metal powder as explained above. In this case, the coating layer can be formed by a compound made of at least one element as its ingredient with a greater affinity to oxygen than that of Fe. By forming a coating layer, it is possible to add acid-resistant, insulation and non-cohesion properties to the magnetic metal powder.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.